1. Field of the invention
The present invention relates to a high capacity and high performance Zr-based hydrogen storage alloy for secondary cells. More particularly, the present invention relates to a Zr-based hydrogen storage alloy for use as an active anode material for Ni-metal hydride (MH) secondary cells.
2. Description of the Prior Art
A Nixe2x80x94MH secondary cell employing a hydrogen storage alloy as the active material of its anode, functions according to the following reaction principle. Upon the discharge of the cell, hydrogen atoms within the hydrogen storage alloy combine with the hydroxide ions (OHxe2x88x92) in a KOH electrolyte to produce water while releasing electrons from the hydrogen atoms via an external circuit to the cathode. On the other hand, upon the charge of the cell, water is decomposed into hydrogen ions (H+) and hydroxide ions (OHxe2x88x92). These hydroxide ions remain in the electrolyte whereas the hydrogen ions combine with the electrons introduced from outside of the electrode to produce hydrogen atoms which are then stored in the alloy, In principle, a Nixe2x80x94MH secondary cell takes advantage of the hydrogen storage alloy""s characteristic properties, including stability in alkali solutions and ability to rapidly absorb and release hydrogen, reversibly.
There are two requirements which hydrogen storage alloys must meet in order for them to be used as active anode materials for Nxe2x80x94MH secondary cells. First, they must have hydrogenation reaction properties, including a hydrogen absorption-release pressure suitable for gas-solid reaction (ordinary, 0.01-1 atm at room temperature), a high hydrogen storage capacity (the theoretical discharge capacity of an electrode is proportional to hydrogen storage capacity (CH wt %): theor. capa. (mAh/g)=268xc3x97CH), and rapid hydrogenation speed. Second, upon the electrochemical reaction of the alloys with KOH electrolytes, the charge transfer reaction, which is closely correlated with the decomposition and synthesis of hydrogen, must readily occur at the interface between the alloys and the electrolyte. That is, the surfaces of the alloys must perform the catalytic function of the charge transfer reaction.
Today, many hydrogen storage alloys have been developed to have the above properties. They can be divided largely into two groups: AB5 and AB2 types. Representative AB5 type includes Laxe2x80x94Ndxe2x80x94Nixe2x80x94Coxe2x80x94Al (see, U.S. Pat. No. 4,487,817) and Mmxe2x80x94Mnxe2x80x94Nixe2x80x94Coxe2x80x94Al (Jap. Pat. Publication Nos. 61-132,501 and 61-214,361), both in hexagonal structure. As those belonging to AB2 type, Tixe2x80x94Vxe2x80x94Nxe2x80x94Cr, a C14, 15-hexagonal, BCC multi-phase t. structure, (see, U.S. Pat. No. 4,551,400), Zrxe2x80x94Vxe2x80x94Ni, a C14 structure (see, J. of the Less-Common Metals, 172-174:1219(1991)), and Zrxe2x80x94Crxe2x80x94Mnxe2x80x94Ni, a C14, 15 structure (refer to supra, 1211(1991)) are exemplified.
Of them, the Laxe2x80x94Ni electrodes of AB5 type are found to show that their electrode capacities are significantly decreased as the cycle of charge and discharge goes on (see, J. of the Less-Common Metals, 161:193 (1990) and 155:119(1989)). This phenomenon, so-called xe2x80x9cdegradationxe2x80x9d, can be solved in the alloy of U.S. Pat. No. 4,487,817 to J. J. G. Willems et al., in which the Ni element is partially replaced by Co and Al and the La element partially by Nd. This technique suggested can improve the life span of the electrode, but causes a decrease in discharge capacity.
Separately, a method of electroless plating Cu on hydrogen storage alloy powders was disclosed in J. of the Less-Common Metals, 107:105(1985), whereby the electrodes can be improved in life span without reduction in capacity. However, this method is complicated and produces pollution of the environment on account of the plating processes and reagents used.
It is now found that AB5 type hydrogen storage alloys show a discharge capacity limit to approximately 300 mAh/g whereas AB2 type hydrogen storage alloys have a discharge capacity greater than 300 mAh/g. In addition, the AB2 type hydrogen storage alloys are also found to be of superior cycle life span even though no plating process is performed (see, J. of the Less-Common Metals, 172-174:1175(1991) and 180:37(1992)).
Zr-based hydrogen storage alloys with a discharge capacity of 300-370 mAh/g, which comprise at least 30 wt % of Zr element and at least 40 wt % of Ni element, are disclosed in U.S. Pat. No. 4,946,646 by T. Gamo et al.
Tixe2x80x94Zrxe2x80x94Vxe2x80x94Nixe2x80x94Cuxe2x80x94Mnxe2x80x94M (M=Al, Co, Fe, etc.) hydrogen storage alloys ranging, in discharge capacity, from 300 to 380 mAh/g are disclosed in U.S. Pat. No. 4,849,205 by K. Hong and in U.S. Pat. Nos. 4,728,586 and 4,551,400 by M. A. Fechenko et al.
These conventional hydrogen storage alloys are disadvantageous in that their rate capability abruptly deteriorates as the discharge current density increases.
Intensive and thorough research repeated by the present inventors aiming to overcome the above problems encountered in the prior Zr-based hydrogen storage electrodes resulted in the finding that a modified Zrxe2x80x94Mnxe2x80x94Vxe2x80x94Ni quaternary alloy, in which the Zr is partially replaced by Ti and the other elements are optionally adjusted, has great hydrogen storage ability and a high discharge capability.
It is therefore an object of the present invention to provide a high capacity and high performance Zr-based hydrogen storage alloy, which is suitable for use as an active anode material for Nixe2x80x94MH secondary cells.
In accordance with the present invention, there is provided a hydrogen storage alloy for Nixe2x80x94MH secondary cells, represented by the following formula I:
Zr1xe2x88x92xTix(MnuVvNiy)zxe2x80x83xe2x80x83I
wherein, x, u, v, y and z each represent an atom fraction under the condition of: 0 less than x xe2x89xa60.2, 0.5xe2x89xa6uxe2x89xa60.7, 0.5xe2x89xa6vxe2x89xa60.7, 1.0xe2x89xa6yxe2x89xa61.4, and 0.84xe2x89xa6zxe2x89xa61.0.